To obtain a silicic acid fertilizer having a high content of soluble silicic acid capable of solubilizing the slag manufactured in a molten iron pretreatment and to provide a method for manufacturing the silicic acid fertilizer.

CaO is added to molten iron and the molten iron is subjected to desiliconizing and dephosphorizing treatments by blowing oxygen thereto, be which the slag having basicity (CaO/SiO2) of ≤3 and containing 15 to 35 mass% soluble silicic acid is batched off and the silicic acid fertilizer is manufactured.